Composition boards like particle boards or other substrates with a thin, hard, formic-like top surface covering are well known. Typically, the surface comprises a cured thermoset resin in a translucent, opaque or printed sheet which is secured to one surface of the board to provide a wear- and stain-resistant surface, such as, for example, in kitchen counter tops, wall coverings, furniture and the like.
A prior art standard method (the high-pressure method) of preparing such composition boards has been to prepare a thick resin-cured laminate overlay sheet typically with four or more underlay sheets of resin-impregnated kraft paper. The overlay sheet is prepared under high pressure and resin-curing temperatures, and the back surface of the laminate sanded or roughened. The laminate, for example, with a conventional thickness of about 0.35 inches, is then glued by means of low or contact-pressure adhesives to the composition board surface. This method requires multiple underlay sheets to provide a cured laminate overlay of sufficient strength to be handled and sanded.
Another technique for producing such composition boards (the low-pressure method) is also used, but likewise has several disadvantages.
In one method, a board core; that is, usually a thick layer of wood chips bonded together by a resin with a thickness ranging from about 0.25 cm to 5.0 cm or more, is used as a base, and a thin, transparent, opaqued or printed sheet, which is impregnated with a curable thermoset resin, such as a melamine-formaldehyde resin, is bonded to the top surface of the board by subjecting it to heat and pressure sufficient to effect simultaneously the curring or fusing of the melamine-formaldehyde reins in the sheet.
In such operation, care must be taken not to employ too much pressure, since pressure, for example, in excess of 10 or 20 kilos per square centimeter or above, often alters or weakens the mechanical and structural properties of the underlying board. In any event, the use of such pressure always reduces the thickness of the board in some degrees. Care must be taken also in the selection of the board, since a board usually has a relatively rough surface created by larger particles or chips of wood and, for example, of more than about 0.3 mm.+-. variation in thickness, would be unacceptable, since the underlying surface defects of the board may be telegraphed through the thin sheet to cause undesirable ripples and surface defects on the top cured surface.
Attempts have been made to overcome this defect by employing wood chips of particularly graded particle size on the surface of the board, but this is expensive and involves another process step. In addition, efforts have been made to smooth; e.g., to presand or presurface, the board surface to make it more acceptable and to raise it into a highly-quality surface board, which is also expensive.
A method of overcoming the problems of telegraphing has been to select a very short heat cycle and to restrict the amount of pressure involved; for example, a heat cycle of only 30 to 90 seconds, and a pressure typically below 20 kilos per square centimeter, such as 5 kcm or lower. By this operation, telegraphing and deterioration of the particle board by heat is minimized and reduction in thickness of the board is minimized also. The top surface of the board prepared in this manner, however, does not present a fully cured resin surface on the thin sheet, but rather a less than fully cured thin top sheet which is more porous in nature than a fully cured sheet, which is subject to high heat and high pressure and longer curing times. The top surface, by being more porous and not fully cured, does not have the high-heat and temperature-resistant properties which would be desirable, and such top surfaces exhibit a reduced resistance to chemicals and stains and a low-resistant surface to abrasion, wear and scratches which is unsatisfactory for many uses.
A further technique employed to overcome the problems associated with telegraphing by the rough surface of a board has been to employ a relatively thick kraft or alpha paper which is impregnated with a thermoset curable resin as an underlay between the thin resin-impregnated top sheet and the rough surface of the board. By this process, lower-quality or rough-type particle boards can be employed and telegraphing prevented. One difficulty associated with the technique is that it involves extra expense and cost through the use of the kraft or alpha paper underlay, and further there is often a visible "show-through" of the phenolic resin impregnated in the kraft paper. In addition, although this technique prevents telegraphing, it still does not permit the use of very high pressures nor the preparation of a fully cured and high stain-, scratch-, abrasion- and wear-resistant top surface in the low-pressure process.
An economic disadvantage associated with the low-pressure process is that any surface defects discovered in the thin top surface result in the discarding of the entire board or at least a reduction in size or trimming of the entire board. Therefore, it is most desirable to prepare a composition board having a thin, fully cured, polymeric, thermoset skin without the disadvantages of the foregoing low- or high-pressure techniques, and which board with the thin skin will exhibit high-heat temperature, high stain, scratch and abrasion resistance.